The imposition of a compositional gradient across a piece of material results, expectedly, in a concomitant gradient in the properties of this material. Such functionally graded materials, or FGM, have recently been the focus of intense investigations, primarily in Japan. Although the initial emphasis was on the synthesis and processing of thermal barrier material for space applications {T. Hirano, Second Symposium for Functionally Gradient Materials, Jul. 1, 1988, Tokyo, The FGM Research Society, Kino Zairyo 8:15 (1988)}, e.g., the National Space Plane and shuttle engines, subsequent investigations have focused on other areas in which the application of FGM provides novel and effective solutions to existing materials problems. These latter areas include the use of FGM in nuclear fusion and fast breeder reactors (as first wall composite materials) {M. Seki, ibid, Kino Zairyo 8:7 (1988); T. Igari, et al., Proceedings of the First International Symposium on FCM; M. Yamanouchi, et al. (eds.), p.11 (1990)}, in electronic and magnetic applications (electro-ceramics, sensors), in optical applications (high performance laser rods, optical disks), in chemical applications (membranes, catalysts), in biomedical materials (tooth implants, artificial bones), and in joining applications (ceramic engines, heat and corrosion resistance coatings) {M. Niino, Kino Zairyo 7:31 (1987); T. Kawai, et al., Proceedings of the First International Symposium on FGM. M. Yamanouchi, et al. (eds.) p. 191 (1990); M. Yuki, et al., ibid, p. 203; M. Chigasaki, et al., ibid, p. 269}. The use of FGM in heat applications is exemplified by the proposed utilization of a TiB.sub.2 /Cu FGM in a reusable rocket engine {T. Hirano, supra}. The ceramic side (TiB.sub.2) of this FGM is designed to withstand 1500 K while the metallic (Cu) side is designed for operation at 300 K The advantage of a functionally gradient TiB.sub.2 /Cu material relative to two layer (TiB.sub.2 +Cu) and three layer (TiB.sub.2 +50% TiB.sub.2 +Cu) alternatives is demonstrated by the results shown in FIG. 1 {T. Hirano, supra}. In cases of a multi-layer system, tensile stresses are generated at the interfaces while, in contrast, the FGM material experiences basically a compressive stress throughout. The existence of tensile stresses at the interfaces in the multi-layer systems is the primary cause of their failure.
To provide a continuous or semi-continuous compositional gradient in functionally graded materials, several synthesis methods have been utilized. These include chemical and physical vapor deposition (CVD and PVD) {S. Ikeno, Second Symposium for Functionally Gradient Materials, Jul. 1, 1988, Tokyo, The FGM Research Society, Kino Zairyo 8:19 (1988); T. Kirai, FGM News, Tokyo, May 1988, p. 10; M. Sasaki, et al., Proceedings of the First International Symposium on FGM, M. Yamanouchi, et al., (eds.) p. 83 (1990); K. Fritscher, et al., ibid, p. 91; S. Uema, et al., ibid, p. 237}, thermal and plasma spray {T. Fukushima, Second Symposium for Functionally Gradient Materials, Jul. 1, 1988, Tokyo, The FGM Research Society, Kino Zairyo 8:31 (1988); H. Steffens, et al., Proceedings of the First International Symposium on FGM, M. Yamanouchi, et al. (eds.), p. 139 (1990); T. Fukushima, et a., ibid, p. 145; N. Shimoda, et al., ibid, p. 151}, powder metallurgy techniques {N. Tsutsumi, ibid, Kino Zairyo 8:39 (1988); R. Watanabe, FGM News, Tokyo, May 1988, p. 14; B. Ilschner, Proceedings of the First International Symposium on FGM, M. Yamanouchi, et al. (eds.), p. 101 (1990); R. Watanabe, et al., ibid, p. 107}, and self-propagating exothermic reactions {N. Sata, Second Symposium for Functionally Gradient Materials, Jul. 1, 1988, Tokyo, The FGM Research Society, Kino Zairyo 8:35 (1988); Y. Matsuzaki, et al., Proceedings of the First US-Japan Workshop on Combustion Synthesis, Jan. 11-12, 1990, Tsukuba Science City, Y. Kaieda, et al. (eds.), National Research Institute for Metals, Tokyo, p. 89; N. Sata, et al., ibid, p. 139; Y. Miyamoto, et al., ibid, p. 173; N. Sata, et al., Combustion and Plasma Synthesis of High Temperature Materials, Z. A. Munir et al., (eds.), p. 195. VCH Publishers, NY (1990); Z. Y. Fu, et al., Proceedings of the First International Symposium on FGM, M. Yamanouchi, et al. (eds.), p. 175 (1990); N. Yanagisawa, et al., ibid, p. 179}. Compared to others, the method of self-propagating exothermic reactions has the general advantages of simplicity, low cost, and the relative ease of preparing larger items. In this method, layers of reactants with gradually changing compositions are pressed together and then ignited at one end of the multi-layer ensembles to initiate a self-sustaining reaction front. The product then comprises regions in which the composition is constant but is incrementally different from that in the two adjacent layers. Thus, the composition changes in a step fashion from one end of the sample to the other.
Despite its attractive features, this method suffers from two general disadvantages. The use of reactant layers with a successively changing composition can lead to the existence of discontinuities in composition at the interfaces. The second disadvantage relates to the nature of the self-propagating reaction: a compositional limit exists at which the reaction enthalpy is not sufficiently high to sustain the combustion wave {Z. A. Munir, American Ceramic Society Bulletin, 67:342 (1988); Z. A. Munir, et al., Materials Science Reports, 3:277 (1989); N. Sata, et al., Proceedings of the Fifth Symposium on High Temperature Materials Chemistry, Oct. 14, 1990, Seattle, Wash., W. Johnson, et al. (eds.), The Electrochemical Society}. This implies in the case of the TiB.sub.2 -Cu system, for example, that a gradient from pure TiB.sub.2 to pure Cu cannot be established by combustion synthesis. At higher copper contents, the reaction Ti+2B+Cu is not self-sustaining. To mitigate the first problem, thinner layers with small compositional differences can be used to prepare the FGM, but this solution adds complexity to the process and diminishes one of its attractive features. The second problem, on the other hand, does not have a simple solution.
The prospect of inducing deliberately designed compositional gradients in optical and electronic materials is extremely attractive in the synthesis of special materials. For instance, gradient index (GRIN) optical elements are now a well accepted part of modern photonic and communication devices. These materials have a well controlled and continuous change in the refractive index and find applications in fiber optic couplers, photocopiers, miniaturized optical systems, and medical endoscopes. The effectiveness of such GRIN elements is strongly determined by the extent of the radial or axial refractive index change (.DELTA.n) that can be obtained in bulk disks or cylindrical preforms of the optical component. GRIN lenses with large refractive index variations (.DELTA.n&gt;0.1) and low dispersion are sought as optical blanks for processing of components with a variety of profiles and symmetry.
In conventional ion exchange processes {S. N. Houde-Walter, et al., Applied Optics 25:3373 (1986)}, a glass rod of homogeneous composition is treated in a salt bath (NaNO.sub.3 +NaCl) to allow Na.sup.+ exchange at the surface. Thus, a surface layer will have a slightly different composition relative to the bulk and this in turn alters the refractive index. The range of .DELTA.n values obtained in this process is of the order of .apprxeq.0.01 to 0.05. For example, TiO.sub.2 -SiO.sub.2 rods, 2 to 3.5 mm in diameter, have been used to obtain lenses with .DELTA.n=0.015 to 0.025. In addition to CVD and sol-gel processing {M. Pickering, et al., ibid, 25:3364 (1986); T. Edahiro, et al., U.S. Pat. No. 4,528,010, Jul. 9, 1985}, other methods for the preparation of materials with a significant .DELTA.n include the co-melting of layers of powdered glasses of different composition to synthesize bulk optical quality materials with .DELTA.n=0.3 to 0.5 {R. Blankenbecler, et al., Journal of Non-Crystal Solids, 129:109 (1991)}. The composition profiles are not very smooth, however, and the process is completely dependent on adequate precision during the mechanical layering of the powders and melt interdiffusion under normal gravity conditions.
Centrifugally-assisted FGM processing is a new synthesis technique for the preparation of these GRIN optical elements, especially since it permits layering of the constituents of the powdered batch under a strong gravity field (50 g.sub.o). The resulting GRIN material is continuously graded in composition and refractive index and the possibility of creating large .DELTA.n in bulk samples, through the innovative use of centrifugally assisted FGM processing, is a major advance over current synthesis methods for GRIN optical materials.
Another important area in which FGM processing is desirable is quantum dot materials. In quantum dot materials, the band gap of a bulk semiconductor is shifted significantly by reducing its particle size to a value smaller than the exciton (electron-hole pair) Bohr radius. The most exciting possibility here is the potential for tailoring quantum devices by size-selection of the semiconductor in nanometer dimensions. Bulk quantum dot materials are composites of semiconductor particles (e.g., CdSe, GaAs) suspended in a ceramic or glass medium. The quality of quantum confinement, i.e., the optical density, is related to the concentration of semiconductor particles and it is here that FGM processing can result in some unique materials with a gradient in semiconductor quantum dot concentration. FGM processing also has the potential for creating quantum-confined structures with predictable quantum dot concentrations in various regions of the same bulk sample. The study of the optical absorption spectra of such samples can be expected to give useful insights into the band structure transitions in these materials. It will be particularly interesting to investigate the non-linear optical response in a bulk solid processed by FGM as the non-linearity can be investigated point to point by moving the laser beam across areas of variable optical density.
A central feature of the method of the present invention is the use of a centrifugal force (F.sub.c) to prepare graded materials with controlled, continuous compositional gradients. The use of a centrifugal force in self-propagating synthesis has been shown to modify the process itself as well as the macroscopic characteristics of the product. However, the primary focus of the use of a centrifugal force has been, thus far, the separation of the product phases to obtain cast materials {A. G. Merzhanov, et al., Nauchnye Osnovy Materialovedniia, Moscow, p. 193 (1981 )}. For the centrifugal force to have an effect, a product or an intermediate phase should be in the liquid state and the phases to be separated should differ in their specific gravity. An example of such a system is the thermite reaction: EQU Fe.sub.2 O.sub.3 +2Al=Al.sub.2 O.sub.3 +2Fe (1)
The adiabatic temperature (3400 K) for this reaction exceeds the melting points of both product phases. However, under actual experimental conditions (where adiabatic conditions are not maintained) or with the addition of appropriate amounts of a diluent (in this case, Al.sub.2 O.sub.3), the combustion conditions give rise to the formation of solid Al.sub.2 O.sub.3 and liquid Fe. Under such conditions, the application of a centrifugal force, F.sub.c, leads to the separation of the lighter phase Al.sub.2 O.sub.3 (.rho.=3.5 g. cm.sup.-3) to the top of the heavier phase, Fe (.rho.=7.86 g. cm.sup.-3). This process has been commercially utilized in Japan to place a corrosion-resistant coating (Al.sub.2 O.sub.3) on the inside of steel pipes {O. Odawara, Japanese Patent # JP55-341416 (1980); O. Odawara, Combustion and Plasma Synthesis of High Temperature Materials, Z. A. Munir, et al. (eds.), p. 179, VCH Publishers, NY (1990)}.
Several investigations on the effect of a centrifugal force on the process of self-propagating synthesis have been made {A. G. Merzhanov, et al., Proceedings of the First US-Japan Workshop on Combustion Synthesis, Jan. 11-12, 1990, Tsukuba Science City, Y. Kaieda, et al. (eds.), National Research Institute for Metals, Tokyo, p. 1}. However, the emphasis of these experimental studies has been the determination of the effect of F.sub.c on the parameters of the combustion process. No systematic study involving theoretical fundamentals of phase separation appears to have been made and no attempt has been made to use a centrifugal force to prepare functionally graded materials with controlled compositional distributions.
An example of the effect of F.sub.c on the velocity of the combustion wave of the reaction described by Eq(1) is shown in FIG. 2 {B. B. Serkov, et al., Fiz. Gor. Vzryva, 4:600 (1968)}. For this reaction, the wave velocity increases almost linearly with increasing F.sub.c (in this figure, F.sub.c is represented by the ratio of the acceleration, g, to that of gravity, g.sub.o). This effect, however, is not universal and is a function of two factors with opposite effects: (a) an increase in rate due to gravity-enhanced permeation of the liquid phase ahead of the wave, and (b) an increase in heat loss due to increased convection in the liquid phases in the combustion zone. The net influence of F.sub.c depends on the relative contribution of these two factors. In highly exothermic reactions the velocity, u, increases dramatically with increasing F.sub.c (curve a), and in moderately exothermic reactions the influence of F.sub.c on u shows a maximum (curves b and c), as seen in FIG. 3 {S. A. Karataskov, et al., ibid, 6:41 (1985)}. The maximum is a consequence of the change in dominance of the two factors discussed above. At lower values of F.sub.c the process is dominated by liquid permeation and at higher values of F.sub.c it is dominated by heat loss due to convection. The importance of a gravitational force on combustion synthesis reactions is underscored by observations that for weakly exothermic reactions, a self-propagating wave can only be established under the influence of F.sub.c (curve d in FIG. 3) and for moderately exothermic reactions (those exhibiting a maximum in u vs F.sub.c) increasing the gravitational force can lead to the extinction of the combustion wave (curve b).
Phase separation as a result of the application of F.sub.c depends on the strength of the force F.sub.c, the viscosity of the liquid phase, the size of the solid particles being separated, the difference between the densities of the liquid and solid phases, and the time during which the centrifugal force is in effect. Since the influence of this force is only seen when a liquid phase is present, the parameter of time, therefore, is dictated by the nature of the combustion process. FIG. 4 shows a schematic representation of the temperature profiles of two combustion reactions in which the effective times are indicated as t.sub.1 and t.sub.2 for the highly exothermic reaction 1 and the less exothermic reaction 2, respectively. In general, phases in systems with higher combustion temperatures can be separated easier than those with lower combustion temperatures. Since the combustion temperature can be controlled by dilution {J. B. Holt, et al., Journal of Materials Science, 21:251 (1986)}, the effective time of the application of F.sub.c can be experimentally varied.
Experimental observations on the effect of F.sub.c on phase separation are shown in FIG. 5 {Merzhanov, et al., supra). It is significant to note that the transition between "no separation" and total separation depends on the system investigated and can take place over a relatively narrow range of F.sub.c values (for the reaction WO.sub.3 +CoO+Al+C) or over a wide range of F.sub.c (for the reaction WO.sub.3 +Al+C).
In a recent investigation {J. B. Hurst, NASA Technical Memorandum 102004, May 1989}, the effect of gravity on phase separation in products of combustion synthesis of nickel aluminides was examined. Cylindrical samples were ignited, in one case at the bottom and the other case at the top. Although no phase separation was detected (under 1 g.sub.o gravity), an interesting observation was made with regard to pore formation. While the total porosity of both types of samples was the same, those that were ignited from the bottom had markedly larger pores when compared to those samples ignited at the top. Qualitatively, these observations suggest that pore segregation occurs in the combustion (liquid) zone. In the case where the wave is propagating in an upward direction, pores that segregate to the top of the combustion zone are swept in the same direction as the wave and coalescence with other pores is likely. With a downward moving wave, pores that segregate to the top of the combustion zone are left behind with little likelihood of growth by coalescence. The effect of a gravitational force of higher than 1 g.sub.o on the size and distribution of pores in this or other systems was not investigated.